@julian
Querying mongodb,
I had a stray category 15 that looked like the following:
{
"_id" : ObjectId("56bd923fc113b09c466f9055"),
"_key" : "category:15",
"parentCid" : 0
}
I removed that entry from mongo and now the API returns a 'NULL' at the end of my categories API.
Is there something else I should delete?

Use custom css in the admin appearance tab like this, using the actual category number you want to hide:
.page-category-16 [component="category/post/guest"] {
display: none;
}
Really, I feel like the Login to Post is for posting to the forum in general, not necessarily to the category being viewed.